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Rochester Question
#1374610 Mon Aug 24 2020 01:11 AM
Joined: Mar 2020
Posts: 78
N
Shop Shark
***Edit: Leaving the question up for reference, but the problem is solved. Not a carb problem, just a bad vacuum leak at the intake.***

Original Question:
Rochester "RS" single barrel according to the stamps on the side; I presume it's still a "B"? Has an Autoline rebuild tag labelled C900,

The engine is a 1957 full pressure 235 (in a '49 3600 but the engine seems to be all '57).

It idled when I first got it, but was full of crud from sitting. The fuel tank and lines are new, and regardless I have good pressure and clean new fuel not only AT the carburetor but IN the carburetor: Float bowl fills up just fine. Accelerator pump SQUIRTS FUEL. But this cursed abomination refuses to FEED FUEL FROM THE BOWL TO THE INTAKE. If I pour gasoline into the throat, the truck starts & runs until that gets eaten up - and by repeatedly pumping the gas I can sort of keep it going.

I can blow air through every passage, took the body and base apart, looking at the shop manual drawings carb cleaner/air seems to come out of everywhere it's supposed to. That includes that passage in the casting running from the main jet at the float bowl to the base by the butterfly.

Power valve is free and clean. New base gasket and insulator although I can't keep it running long enough to say for sure I don't have a vacuum leak somewhere.

Thoughts? This is clearly something dumb that I'm missing. Thanks folks...

Update: I got to thinking perhaps the float level was out of spec (I hadn't actually checked it). It WAS out of spec; adjusted but there was no change to this condition.

Last edited by NorthCoast3800; Wed Aug 26 2020 12:09 AM. Reason: Update

1949 Chevrolet 3/4 Ton - Still Solid.
Re: Rochester Question
NorthCoast3800 #1374658 Mon Aug 24 2020 01:37 PM
Joined: Nov 2019
Posts: 64
H
Wrench Fetcher
Real simple thing, but you did ensure the bottom gasket is on correctly and not covering the passage holes from the bottom of the bowl to the underside of the butterfly? Idle mixture screw is out and not screwed all the way in right? You don't happen to have a spare carb laying around to test with do you?

Re: Rochester Question
NorthCoast3800 #1374669 Mon Aug 24 2020 04:49 PM
Joined: Jan 2007
Posts: 199
R
Shop Shark
Based on your description of the issue you are experiencing. ...is this a good summary? ... This carb RAN on this engine at at idle. Then something changed, you took it off, cleaned it, did some fuel system confirmation and now it will not use gas from the bowl. Did you partially rebuild it or use new gaskets? Something changed and that is what I would be first looking at to establish a starting point. In my recent adventure with a mono jet Rochester, a new fuel pump was my problem....pushing too much pressure and creating flooding that made me think my carb work was suspect. I went back to the beginning after several carb tinkerings and finally checked the "NEW" part and found the problem.

Whatever you did from running to not running should be reviewed. Seems like your description has the carb isolated as the engine runs when you hand feed and bypass the carb. I wonder if you start it with a helper with fuel and immediately go to revving off the idle circuit and will the accelerator pump try and let it run to narrow down the potential problem? Otherwise if that carb won't allow any fuel through it, not enough vacuum ( massive leak) or blocked carb passage? I would think it would have to be a big vacuum loss to not allow ANY fuel to be drawn down and evident. If you are fairly certain of vacuum, that carb has to be really inspected taking nothing for granted with just passing air through. Good luck.


Rasman 1971 Chevy C10 stock 260 6 cyl 3 on the tree manual everything.
1959 Chevy 1/2-Ton [stovebolt.com]


Re: Rochester Question
NorthCoast3800 #1374700 Mon Aug 24 2020 07:34 PM
Joined: Mar 2020
Posts: 78
N
Shop Shark
Thanks for the replies; HF Farms that's a good point, I double checked the gasket just now and there is indeed a cutout for that passage by the butterfly. It has a new insulator base so I just checked that for flat with a machinist straight edge, can't see any light on either side (flat). Resinstalled, no change.

Rasman - Yes it ran at idle. I fully agree, and I'm trying to nail it down hoping someone's had a similar misadventure. I haven't rebuilt it (yet) only cleaned it out, the gasket for the top part didn't tear and I happened to have a new gasket for that base to bowl. Checked them for flat as well. All the passages blow clear which is what's driving me nuts, if the main jet was plugged I could see this happening. The idle passage from the main jet in particular, is free right from the jet down to the cast iron base. I've pulled it apart several times now and check that every time.

Original fuel pump, only the tank and lines were replaced. Fuel feed is fine and with the original pump, the pressure would not have increased.

Highly annoying. I'm guessing vacuum leak but would assume the thing would still run - even choking it out won't allow it to draw fuel into the intake from the bowl. Dribble some gas in there, it starts right up. As I say, hit the throttle and fuel squirts out. But it will not come out from the idle circuit. Again, importantly, there is fuel flowing into the bowl (i.e. the needle is not stuck); float level is correct.

It will indeed be something obvious when I find it....


1949 Chevrolet 3/4 Ton - Still Solid.
Re: Rochester Question
NorthCoast3800 #1374721 Mon Aug 24 2020 10:12 PM
Joined: Mar 2020
Posts: 78
N
Shop Shark
Seems like a severe intake leak. I tightened everything up and cranked up the timing and it will actually run right now on by maintaining some throttle. We'll get there.

Update: Re-reading my original post I realized I omitted the little detail that I've replaced the intake/exhaust gasket. I hadn't taken the time to read up on that little procedure before I did it, just figured those 3 rings would do their thing on their own. So I've re-done it, hopefully that gets it. The holes in the head are larger than the holes in the intake so hopefully that's not an issue (could be a '49 intake on a '57 head who knows).

Anyway, I'll update tomorrow if it works gotta get back to the paying job.

Last edited by NorthCoast3800; Mon Aug 24 2020 11:58 PM. Reason: Update

1949 Chevrolet 3/4 Ton - Still Solid.
Re: Rochester Question
NorthCoast3800 #1374739 Tue Aug 25 2020 12:32 AM
Joined: Aug 2018
Posts: 272
D
Shop Shark
If you continue to have alignment issues on the intake manifold, consider swapping out the standard rings with the adapter rings readily available on eBay and elsewhere that will adapt 216 and 235 port sizes. Here's an example (no affiliation, just an example): 216 to 235 intake adapter rings [ebay.com].

Doug

Re: Rochester Question
NorthCoast3800 #1374741 Tue Aug 25 2020 12:37 AM
Joined: Mar 2020
Posts: 78
N
Shop Shark
DrDoug, thanks for that!


1949 Chevrolet 3/4 Ton - Still Solid.
Re: Rochester Question
NorthCoast3800 #1374781 Tue Aug 25 2020 01:59 PM
Joined: Dec 2017
Posts: 822
D
Shop Shark
You should definitely consider using the adapter alignment rings. Also, consider using a Rimflex gasket. its one piece and while its three times as thick as a stock gasket before installation, it compresses down and really does a terrific job sealing the intake and exhaust.


Mike
Re: Rochester Question
NorthCoast3800 #1374789 Tue Aug 25 2020 03:09 PM
Joined: Mar 2020
Posts: 78
N
Shop Shark
Thanks; unfortunately their eBay store doesn't ship to Canada and I don't see them elsewhere. HOWEVER I have a lathe here and a selection of pipe, if she leaks again I will turn a few to fit on-site.

Good tip on the Rimflex I'll source some so I have them on hand!

NC3800


1949 Chevrolet 3/4 Ton - Still Solid.
Re: Rochester Question
NorthCoast3800 #1374809 Tue Aug 25 2020 06:33 PM
Joined: Jan 2020
Posts: 533
D
Shop Shark
As is the usual case with this site you are receiving quality information. I'm Known as the Rochester B, BC & BV Nazi in my local area. Something that many miss on the rebuild is the power piston. once you take the air horn (top of carb., four screws), check the power piston, this is the tube that looks like a ball point brass refill. Very carefully pull it out, if is hard to move soak well wth Krol oil or Blaster etc. NEVER WD-40 (it will crystallize under heat). Twist very gently and pull base with needle nose pliers. You do not want to break it, it cannot be ordered. there is a small spring under it so don't loose it either. I always polish the chamber and stem with a small round gun brush. This has to move up and down, if it is not free it can cause all kinds of problems. Also be sure the the accelerator pump is not sticking and is clean, gently brush out the well and put the right number of balls in the well pending carb casting. Doc


Currently making 1954 3100 better than new
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